The present invention generally pertains to chemical vapor sensors and related control circuitry, and more particularly to chemical vapor sensors having rapid desorption rates so as to provide rapid information concerning the decrease of chemical vapors in an environment.
Adsorption-type sensors sensitive to hydrocarbon containing vapors have been used in various applications for many years. Common applications include the use of such sensors in marine (bilge and engine rooms), underground (wells and storage tanks), and similar applications. The purpose of these sensors is to provide information concerning the presence of hydrocarbon containing vapors to a monitoring system. To this end, these sensors were optimized to quickly detect chemical containing vapors, and especially hydrocarbon containing vapors, through adsorption since ambient conditions were presumptively without the presence of such compound containing vapors.
Examples of these sensors can be found in, for example, U.S. Pat. Nos. 3.045,198, 4,224,595, and 4,752,761, which are incorporated herein by reference. These sensors utilized electrically conductive adsorbent particles that were resiliently attached to a surface so that an electrically conductive path was formed through the particles. When subjected to a vapor containing hydrocarbons, for example, the resistance between the electrodes contacting the conductive adsorbent particles was increased. Improvements to the sensitivity and range of the sensor were achieved by using heterogeneous particles in a given conductive pathway, and providing control circuitry for carefully regulating the temperature of the particles.
As noted above, however, sensors of the type described were optimized to detect hydrocarbon containing vapors in ambient conditions generally lacking such adsorbates. Where the ambient conditions have a high concentration of hydrocarbon containing vapors, or liquid phase hydrocarbon containing compounds, the aforementioned optimizations generally fail. This shortcoming is important in view of the following.
Federal and local government agencies now require that gasoline dispensers be capable of recovering the gasoline vapor that is displaced from an automobile""s tank by liquid gasoline during a refueling operation. This is usually accomplished by means of a vacuum vapor recovery system that works in conjunction with the fuel pumping system. A dual conduit hose and a compound nozzle are used to deliver liquid gasoline to a vehicle""s tank while simultaneously drawing off the displaced gasoline vapors through a separate vacuum port near the tip of the nozzle. The vacuum system returns the displaced vapor back to the service station""s underground storage tank as is best shown in FIG. 1. In such a manner, a system efficiency of approximately 95% can be achieved (Transfer emissions of 0.34 pounds of hydrocarbons per 1000 gallons dispensed; Fugitive emissions of 0.08 pounds of hydrocarbons per 1000 gallons dispensed).
The Environmental Protection Agency (EPA) has recently mandated that beginning in 1998, certain modifications must be made to new vehicle fuel systems to prohibit the displacement of fuel vapors resulting from filling actions to the environment. In essence, these on-board refueling vapor recovery systems (ORVRS) will take the place of a station equipped system, thereby obviating the need for any vacuum recovery system. However, if an ORVRS equipped vehicle is refilled from a vacuum recovery system equipped station, the system will only recover ambient air. While this may initially appear to be of no consequence, industry testing has shown that unacceptable quantities of gasoline vapor escape from the underground tank vents when ambient air is introduced into the vacuum vapor recovery system. As shown in FIG. 2, transfer emissions remain similar to those generated with the vacuum recovery system, however because of in increase in static pressure in the station storage tank, fugitive emissions are-increased by over 30 times. The result is a system efficiency of only 60% as opposed to 95%.
To provide a solution for selectively capturing only gasoline vapor containing air and not unnecessarily introduce ambient air into a station""s fuel storage tanks, it would be most desirable to sense whether a refueling vehicle possessed an ORVRS so that a vacuum recovery system could be temporarily disabled so as to prevent this undesirable introduction of ambient air into the storage tank. Because there appears to be no standard physical configuration relating to the refueling nozzle, such sensing would likely be limited to determining the quality of the evacuated air, i.e., whether it contained a high vapor level of hydrocarbons which are found in all present fuels.
Since only vehicles manufactured and sold from 1998 and forward would be equipped with an ORVRS, the sensing means would have to be able to tolerate frequent exposure to air that was highly saturated with hydrocarbon compounds including solvents such as gasoline or diesel fuel. The sensing means would have to not deteriorate over prolonged exposure to such an environment. Moreover, the sensing means would have to have a rapid response time so that an effective interruption of a vacuum recovery system could be carried out prior to a significant introduction of ambient air into the storage tank. Gasoline dispenser manufacturers estimate that the detection of ambient air in the vacuum recovery system must occur within 10 seconds of its introduction into the vacuum port on the nozzle.
The sensors of the prior art fail to meet these needs since they are optimized to operate in ambient air conditions with only an occasional exposure to destructive hydrocarbon containing vapors. Their response time, as determined by a desorption time factor, fails to indicate the presence of ambient air after exposure to hydrocarbon containing vapor within the recommended 10 seconds. In addition, their longevity when placed in the harsh environment of solvent vapors is questionable at best.
The present invention is drawn to improvements in hydrocarbon adsorption sensors and related circuitry. The improvements relate to the manner of intended use of the sensor, namely use in an environment wherein the presence of the compound being sensed is normal and rapid sensing of conditions lacking the compound is desired. To this end, the sensor of the present invention in a first embodiment comprises an electrically resistive element having a first conductive lead and a second conductive lead; an elastomeric material in thermal contact with the resistive element; a stratum of conductive particles generally adhered to and at least partially covering the elastomeric material to form a sensitive surface having variable electrical resistance properties wherein a first portion of the conductive layer is electrically coupled with the first conductive lead; and a third conductive lead is electrically coupled with a second portion of the conductive stratum physically apart from the first portion.
Features of the sensor that improve its rapid determination of hydrocarbon absence include using 50 micron or smaller conductive particles, using 100% silicone with no adjuncts as the elastomeric material, using the resistive element as a heat source to periodically desorb the adsorbed compounds from the stratum, and utilizing a simple control circuitry to control the resistive element heating cycle. In addition to sensitivity improvements, another feature is directed to improving the longevity of the sensor. This feature is accomplished by tapering at least one portion of the elastomeric material adjacent a resistive element lead so that when an end cap is placed thereat to achieve electrical conduction there between, neither the cap nor the stratum will be undesirably stressed when the elastomeric material expands due to exposure to solvents or the like.
The control circuit for use with the sensor broadly described above is intended to sense the level of stratum resistance and cause desorption therefrom when the sensor reaches a threshold adsorption level. This function is carried out by a negative feedback circuit and comprises a first voltage divider and a second voltage divider operatively coupled to a voltage comparator; and a power amplifier operatively coupled to an output of the voltage comparator and to an electrically resistive element of a sensor having a variable resistance sensitive surface, wherein the sensitive surface comprises a part of the first voltage divider. By this arrangement, power is selectively applied to the power amplifier by the comparator for distribution to the resistive element when a predetermined condition exists between the first and the second voltage dividers. Because a portion of the first voltage divider comprises the sensitive surface, the resistance of the sensitive surface is compared to a reference resistance that is part of the second voltage divider. When the sensitive surface resistance increases beyond a threshold due to excessive adsorption, the comparator causes power to be applied to the resistive element of the sensitive surface to heat the same, and cause partial desorption of the sensitive surface, thereby restoring its ability to rapidly desorb when exposure to fresh air occurs.